1. Field of the Invention
The invention relates generally to acoustical investigating of a borehole, and to the determining of an impedance of a material behind a casing located in a borehole
2. Background Art
Acoustical investigating is widely used to investigate casings found in boreholes. Typically acoustical investigating uses ultrasonic waves.
Boreholes penetrating an earth formation generally comprise a wall with an annular space filed with set cement. After the cement has set in the annular space of the casing it is common practice to use acoustic non-destructive testing methods to evaluate its integrity. This evaluation is of prime importance since the cement must guarantee zonal isolation between different formations in order to avoid flow of fluids from the formations (water, gas, oil) through the annular space of the casing.
Turning now to FIG. 1, a schematic diagram of a logging operation is shown. Tool, or sonde, 10 for evaluating cement quality is located in borehole 11 penetrating earth formation 12. Casing 13 is cemented to the walls of the borehole, as explained in detail with reference to FIG. 3, below. The sonde is preferably lowered in the borehole by armored multi-conductor cable 14 and slowly raised by surface equipment 15 over sheave wheel 16 while cement quality measurements are performed. The depth of the tool is measured by depth gauge 17, which measures cable displacement.
Sonde 10 measures cement quality by emitting an acoustic pulse and analyzing its return waveform. The sonde is capable of obtaining cement quality measurements azimuthally. The sonde measures cement quality by emitting acoustic excitation pulses and analyzing the return waveforms produced as a result of reflections from the casing, as well as reverberations of the casing. The return waveforms can be analyzed by the sonde in situ, analyzed by data processor 18 at the surface, or stored, either in the sonde or at the surface, for analysis at a remote location. In the preferred embodiment, the return waveform data is transferred to data processor 18 by cable 14, where the cement quality is determined.
The excitation pulse preferably excites a thickness resonance of the casing. As is known in the art, such resonance traps energy in the casing. The subsequent reduction of trapped energy in the casing may be considered the result of leakage attributable to the degree of acoustic coupling to adjacent media. All resonances excited by the excitation pulse trap energy in this manner.
It is possible to design a sonde having a plurality of transceivers, each of which generates excitation pulses which excite the fundamental resonant frequency for each of the plurality of different nominal casing thicknesses encountered. In a preferred embodiment, however, the sonde includes one transceiver having an excitation pulse which excites either the fundamental resonance, or a harmonic thereof, for the wide range of casing thicknesses typically encountered. The acoustic pulse is, therefore, preferably highly damped and of short duration on the order of eight microseconds or less. An example of frequency spectrum of the acoustic pulse, is shown with reference to FIG. 2, and has a 6-dB bandwidth of about 400 kHz centered at about 450 kHz.
Turning now to FIG. 3, a typical return waveform from a casing excited by the acoustic pulse of FIG. 2 is illustrated. The return waveform, a typical response from a steel casing of approximately 17.8 cm in diameter, includes initial reflection segment 31 and reverberation segment 32. The initial reflection segment is due primarily to the reflection of the pulse from the inner surface of the casing. The magnitude of the initial reflection segment is a function of borehole fluid (e.g., composition of the mud), casing surface conditions, alignment of the sonde, as well as transceiver output.
The portion of the excitation pulse not immediately reflected enters the casing and excites the resonances in the casing, as discussed in detail above. Thus, the reverberation segment is due primarily to acoustical energy which was trapped in the casing, leaks back into the borehole fluid, and propagates back to the receiver. The relative amplitude and duration of the reverberation segment is a function of the amount of energy transferred from the casing to the formation via the cement therebetween. As is well known, the amount of transferred energy is a function of the characteristics of the cement seal coupling the casing and formation. Good cement quality would transfer more energy than would poor cement quality. Therefore, the casing resonances decay more rapidly for good cement quality than for poor cement quality. The waveform shown with reference to FIG. 3 has relatively rapid decay which is indicative of good cement quality. Lack of cement adhesion produces an error in the quantitative determination of impedance. However, the error in the apparent impedance due to lack of adhesion is small compared to the difference in impedance between typical good cement and that where cement is absent.
The return waveforms may be analyzed by data processor 18 of FIG. 1. U.S. Pat. No. 4,928,269 to Kimbal et al., published on 22 May 1990 discloses an example of a method for calculating the impedance of cement behind the section of a casing.
An impulse response Rcas of the mud/casing/cement system results from reflection coefficients and transmission coefficients at each interface. The reflection coefficients and transmission coefficients at each interface are on turn function of acoustic impedances of the mud, casing and cement. Hence is becomes possible to invert the acoustic impedance of the cement from a known impulse response Rcas.
An actual transducer response W to a casing is a convolution W=Rcas{circle around (x)} T of the impulse response Rcas with a transmit/receive response T of the transducer, as would be recorded by reflection on a single interface with infinite impedance contrast.
Downhole, only W can be recorded and T is not know, since it is affected by mud impedance and attenuation, and also temperature and pressure which change characteristics of the transducer itself, such as the backing impedance. Hence it is necessary to approximate the transducer response T to obtain the impulse response Rcas and invert the cement impedance.
Currently used processing methods for evaluating the impedance of the cement produce strongly biased results when determining the cement impedance.